Updated: Dec 12, 2020
Designing a heated water circulating system is time consuming, tedious, inefficient, and uneconomical. It is also relatively difficult to get correct.
Why? Well, to correctly design a heated water circulating system, you need to follow these six key steps:
1. Keep the System Simple
This sounds like an easy task and it usually is. However, we have to be careful not to design a system that has any well intentioned errors.
'What is a well intentioned error?' is a good question. Generally, this is where a system has been designed with redundancy in mind, but by creating interconnections and rings off rings, we are designing a system that is almost impossible to circulate.
When designing a heater water circulating system - keep it simple.
Below are diagrammatic examples of systems we should and shouldn't be designing.
2. Designing to the Correct Flow Rate
Did you know that some international standards require you to size all of your pipes for a combination of the peak simultaneous flow rate in addition to the heat loss from the pipe.
The reason we are required to do this is because the heated water circulating pump is usually a continuous flow. Whether the system is experiencing a peak demand, no demand, or anything in between, the heated water circulating pump will be demanding the same flow rate in addition to this.
Sizing a pipe based on the peak flow rate is common practice. Sizing a pipe based on the heat loss is also common practice. Sizing a pipe based on a combination is not common practice. This is due to two reasons:
1 - The industry generally appear to be unaware of this requirement
2 - It is very difficult to do as it is an iterative process
The below diagram highlights the iterative process that needs to be followed once you have determined your peak flow rate and need to add the heat loss on top of this flow rate.
Despite it being a difficult task to complete, by neglecting this requirement, you would consequently be designing a system that is susceptible to a non-compliant velocity.
3. Distributing the Heat Loss Flow Rate
Keeping the pipe sizes in a heated water system minimal should always be a priority, this keeps the heat loss in the system to a minimum.
The heat loss flow rate from common pipes, such as the riser, should be distributed economically. Our design calculations should not just simply take the heat loss flow rate solely through one circuit, this can lead to increasing pipe sizes unnecessarily.
To do this effectively, spare capacity in pipes that are uncommon, such as pipework that reticulates on a floor level, should be assessed and the heat loss flow rate should be distributed where minimal impact will be made.
For an exaggerated example of this in a simple system, the below sketch highlights why we should design where the heat loss flow rate goes.
4. Pressure Loss Through the System
Once you know the heat loss flow rate through the system, you need to determine the pressure loss through each circuit.
The pressure loss through each circuit should not be added together. It should only be the pressure loss through the most hydraulically disadvantaged circuit that is used to size the circulating pump.
This can easily envisaged and understood when you compare it to a typical cold water system. When sizing a cold water booster pump, if you calculated the pressure loss to each fixture. then added all of these pressure losses together to determine the pump duty, it would result in a very oversized pump.
As long there is enough pressure to overcome the most disadvantaged circuit, the rest of the system will work too.
5. Balancing Valve Location
Balancing valves are required to create equal pressure loss through all parts of the system once the flow rates through each circuit is known. Without them, water would take the path of least resistance and you would be left with an unbalanced system.
It is imperative that the pressure in the system is equal where circuits interconnect. Therefore, balancing valves should all be set to create different amounts of pressure loss.
If the pressure is not equal where two returns interconnect, you could think of it the same as having two water mains, one with high pressure and one with low pressure. The building would draw more water from the high pressure water main.
Can the balancing valve be upstream of fixtures?
Theoretically yes but generally not fit for purpose.
It is a common misconception within the industry that there can not be a fixture downstream of a balancing valve, but this is not true. Placing a balancing valve upstream of fixtures would have the same effect as placing it downstream in relation to the equal circulation of the system.
However, depending on the flow rate, you may get a problem with pressure loss during a peak demand and this is the reason why it is common practice to locate downstream of fixtures. You would also need to ensure the pressure loss does not have an adverse effect on the circulating pump operation.
Do you need a balancing valve on the most disadvantaged circuit?
No. However, it is sensible to provide one there for future flexibility.
6. Pump Selection and Correct Operation
Once you have your heat loss flow rate and pressure loss, this is known as your system duty. You need to ensure that your system duty matches the pump duty of the selected pump.
This is a very important step of the design that is often overlooked and has the potential to be fatal to a system. It is checked by looking up the selected pump's pump curve.
Note: the system duty is shown with the hollow red circle, the pump duty points are shown with the three blue diagonal lines, and where the system duty meets the pump curve is shown with the red and yellow circle
In the example shown above, note how the system duty does not align with the pump duty. Consequently, the flow rate has increased from the system duty of 0.20 L/sec and risen to 0.30 L/sec.
This causes the velocity to increase and potentially rise to above the compliant threshold. In the above example, the velocity rises from 0.89 m/sec to 1.34 m/sec.
To deal with this, you should create more friction loss in the system. This is best done by increasing the pressure loss through the balancing valves. The additional pressure loss required is the difference between the system duty point and the pump curve directly above.
In this example, we have designed an extra 2mH pressure loss so that our system duty meets the pump curve without increasing the flow rate and velocity.
Additionally, always ensure that the circulating pump is on the correct setting when installed. Even if you design the system perfectly, the installer may set it to the maximum setting and as we just learnt, this will cause you issues.
Variable speed driven circulating pumps are now available which can assist in this area. However, for these pumps to work as required, a temperature gauge should be located on each of the circuits and not just upstream of the pump. Failing to do this will mean that you risk water in your most disadvantaged circuits dropping below the required temperature.
Let us know your thoughts in the comments, are these all necessary steps in your opinion?
Luckily, difficult tasks are being designed out of life in the 21st century. If you use H2X, the design of a hot water circulating system is another task that has now been designed out of your life.
Check out this video to see this design process become fully automated by simply drawing lines.
Time consuming, tedious, inefficient, and uneconomical has become efficient, stress free, and economical.